Research to explore elementary constituents of material in the world
found parton (quarks and gluons) in nucleon by means of
electron-proton deep inelastic scattering (DIS) experiments.
Many physicists drive themselves forward to discover further
fundamental particles and the trend leads to the energy frontier
as represented by LHC. Meanwhile, it is also important challenge
to understand complex QCD phenomena, which governs the dynamics of
quarks and gluons. We are on the latter stream via studying
spin-dependent inner structure of proton using polarized proton beam
of RHIC (Relativistic Heavy Ion Collider) at BNL (Brookhaven
National Laboratory, US). The proton spin structure had been
studied for decades mainly utilizing polarized electron-nucleon
DIS (pol-DIS) before RHIC started measurements in 2002. Contrary
to expectations based on the success of static quark model of
hadrons, the pol-DIS experiments concluded that contribution of
quarks and anti-quarks to the proton spin is only ~25% of the total.

One of the major goals of the PHENIX experiment at RHIC is to
directly determine gluon spin contribution to the proton spin
utilizing polarized proton-proton collisions. The first results
from PHENIX were measurements of double helicity asymmetry (A_{LL})
in pi0 production in 2003, which is connected to the polarized
gluon distribution function in the proton (Delta-g). The PHENIX
experiment has been accumulating data and more than 100~times
larger statistics than the first year are obtained in the latest
2010 run with sqrt{s}=200 GeV. We published improved results of
A_{LL} in pi0 production, as well as other various channels.
The results of RHIC were analyzed in the framework based on
perturbative QCD factorization theorem together with pol-DIS
measurements and current our best knowledge of Delta-g is obtained.

Another challenging goal of the PHENIX experiment is probing
the contribution of flavor-sorted anti-quarks to the proton spin
utilizing Weak boson (W) production. While it's difficult to
distinguish quark flavor with electromagnetic interaction in
pol-DIS, W boson is generated via quark and anti-quark annihilation
and sensitive to quark flavor. The PHENIX detector is capable of
measuring W boson using central arms for W->e decays and
forward/backward muon arms for W->mu decays. We are developing
new W trigger for muon arms to overcome huge background in
W boson measurements. In coming 2011 run, the first long polarized
proton-proton collisions with root{s}=500 GeV is scheduled and
we aim to extract polarized sea-quark distribution functions in
nucleon.

In this talk, we will introduce current results of
RHIC-PHENIX spin program and prospects in the near future.